Method and apparatus for an integrated laser beam scanner

ABSTRACT

A solid state laser beam scanning system having a single crystal silicon deflection and scanning mirror integrated with a laser diode. By combining the techniques of deep reactive ion etching of silicon with solder bump bonding techniques, completed and tested laser diodes are integrated with silicon substrates supporting micro-electro-mechanical systems layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to "METHOD AND APPARATUS FOR ANINTEGRATED LASER BEAM SCANNER USING A CARRIER SUBSTRATE" by Floyd, Sunand Kubby (Attorney Docket No. D/98707), Ser. No. 09/203,442, filed onthe same day and assigned to the same assignee which is herebyincorporated by reference in its entirety.

BACKGROUND AND SUMMARY OF INVENTION

The present invention relates generally to the field of laser beamscanning systems, and more particularly to micro-electro-mechanicalsystems (MEMS) for laser beam scanning. Miniature laser beam scanningsystems are important for applications such as barcode scanning, machinevision and, most importantly, xerographic printing. The use of MEMS toreplace standard raster output scanning (ROS) in xerographic printengines allows simplification of printing systems by eliminatingmacroscopic mechanical components and replacing them with large arraysof scanning elements. Advanced computation and control algorithms areused in managing the large arrays of scanning elements.

Such MEMS based printing systems are entirely solid state, reducingcomplexity, and allowing increased functionality, including compensationof errors or failures in the scanner elements. An important step inconstructing solid state scanning systems is integrating thesemiconductor light emitter directly with MEMS actuators to gain thedesired optical system simplification. Integrated scanners, which havelasers and scanning mirrors in the same structure, have beendemonstrated using manual placement of laser chips onto MEMS wafers withmicromachined alignment parts and adhesives by L. Y. Lin et al inApplied Physics Letters, 66, p. 2946, 1995 and by M. J. Daneman et al inPhotonics Technology Letters, 8(3), p. 396, 1996. However, currenttechniques do not allow wafer-scale integration of the light-emitter andMEMS device.

In accordance with the present invention a laser beam scanner consistingof a single crystal silicon (SCS) deflection and scanning mirror isintegrated with a laser diode or light emitting diode. By combiningmethods of deep reactive ion etching (deep RIE) of silicon with solderbump bonding methods, completed and tested laser diodes are integratedwith silicon (Si) or silicon on insulator (SOI) substrates supportingMEMS layers. Details of creating a torsional mirror and actuating itmagnetically or electrostatically are detailed in U.S. Pat. No.5,629,790 by Neukermans and Slater which is incorporated herein byreference in its entirety.

Using solder bump bonding methods, completed and tested laser diodes arebonded to silicon MEMS built using a typical surface and bulkmicromachining processes. Because of the deep RIE recesses, the laserdiode solder bumps can be passively aligned to those on the hostsubstrate. In addition, the deep RIE recesses allow nearly coplanarlaser chip and Si surfaces to be made. The use of the SCS layer of anSOI wafer, rather than the polysilicon film provides for theintroduction of very flat and smooth mirrors and high reliabilitytorsion bars. The device is easily scalable to arrays of lasers andscanning mirrors on a single wafer.

Integration of the scanner and light source eliminates the need forexternal, manual positioning of light sources and scanning mirrors.Simplified and more cost effective post-processing steps such asinterconnect metallization can be realized because the use of an etchedrecess results in nearly planar surfaces. In addition, pick and placetechnologies commonly used for multi-chip module assembly can be adaptedfor wafer scale assembly and bonding of light sources to the carriersubstrate. With such commercial systems, bare die can be placed withaccuracy better than ±30 μm.

Thus, the present invention allows the integration of completed andtested light emitting devices directly with the MEMS actuators to gainthe desired simplification of the optical system needed to realize solidstate scanning systems.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained and understood by referringto the following detailed description and the accompanying drawings inwhich like reference numerals denote like elements as between thevarious drawings. The drawings, briefly described below, are not toscale.

FIG. 1 shows an embodiment in accordance with this invention of anintegrated solid state scanner and laser.

FIG. 2 shows a top view of an embodiment in accordance with thisinvention.

FIGS. 3a-3c show alternative embodiments in accordance with thisinvention of an integrated solid state scanner and laser.

FIGS. 4a-4n show the processing steps for constructing an embodiment inaccordance with this invention.

DETAILED DESCRIPTION

An embodiment in accordance with this invention is shown in FIG. 1. Alaser beam scanner consisting of a single crystal silicon (SCS)deflecting mirror 140 and torsional mirror 150 is integrated with laserdiode or light emitting diode 105. By combining methods of deep reactiveion etching (deep RIE) of silicon with solder bump bonding methods,completed and tested laser diodes 105 are integrated with silicon (Si)or silicon on insulator (SOI) substrates 100 supporting MEMS layers. Forexample, completed GaAs-based lasers may be joined to Si MEMS to makelaser beam scanners in accordance with this invention.

Electrical contact to laser 105 can be made in a variety of ways.Contact can be made by planar surface metallization, by wire bonding tothe laser, through the polysilicon layer with solder bumps or somecombination of each. Using solder bump bonding methods, completed andtested lasers 105 are bonded to layer phosphorus-doped glass (PSG) layer119 in recess 135. Deep RIE etching may be used to define recess 135 inthe MEMS surface layers 130 and Si substrate 115 for subsequent laserchip 105 placement. Because of deep RIE recess 135, laser diode solderbumps 110 can be passively aligned to wettable metal bonding pads 111 onsubstrate 115. In addition, deep RIE recess 135 allows for nearlycoplanar laser chip 105 and Si surfaces. This allows simplification ofthe subsequent metallization steps and laser chip 105 does not interferewith the space used for the optical path. The use of SCS layer 130 ofSOI wafer 100 for the mirror material, rather than polysilicon filmprovides for the introduction of very flat and smooth mirrors 140 and150 and high reliability torsion bars 170. The device is easily scalableto arrays of lasers and scanning mirrors. The reflective surface ofdeflecting mirror 140 and torsional mirror 150 is typically coated withaluminum 430 (see FIGS. 4e-4m).

In FIG. 1, MEMS components such as single crystalline silicon (SCS)mirror 140 and torsional mirror 150 are formed in SCS layer 130 by usinga combination of well-known surface and bulk micro-machining techniques.VCSEL (vertical cavity surface emitting laser) 105 is solder bump 110bonded to wettable metal bonding pads 111 residing on phosphorus-dopedglass (PSG) layer 119. Additionally, actuation electrodes 120 are formedon glass or SiO₂ coated silicon substrate 101 which is bonded to SCSsubstrate 115 after etching hole 117 (see FIG. 4a). Typical thicknessesare 0.5 mm for SCS substrate 115 and glass or dielectric-coated(typically SiO₂ or SiN_(x)) Si substrate 101. SCS substrate 115 has deepRIE and/or wet etched recess 135 for alignment and placement of VCSEL105 on SCS layer 130. VCSEL 105 typically has a divergence angle ofabout 14 degrees. Emitted light 199 then passes onto deflecting mirror140 which reflects emitted light 199 onto torsional mirror 150. SCSlayer 130 is typically about 2-20 μm thick. The spot diameter of emittedlight 199 at deflecting mirror 140 is typically about 234 μm and thespot diameter at torsional mirror 150 is typically about 480 μm.

Polysilicon hinge 155 is micromachined from a deposited polysiliconlayer and attaches deflecting mirror 140 to SOI substrate 115.Polysilicon hinge 155 allows deflecting mirror 140 to rotate clockwiseabout an axis perpendicular to the plane of FIG. 1, out of SCS layer 130to a typical angle of about 30 degrees above recess 135 as shown inFIG. 1. The distance between polysilicon hinge 155 and torsional mirror150 is typically on the order of 1.1 mm. A typical length for deflectingmirror 140 is about 1 mm. Deflecting mirror 140 can be supported bysupport latch 168 controlled by a spring and latch assembly (not shown)in the manner described by Lin et al. in Photonics Technology Letters,6(12), p. 1445, 1994 and incorporated herein by reference in itsentirety. Typically, the length of support latch 168 is 100 μm.Controlling the position and length of support latch 168 allows theangle of deflecting mirror 140 with respect to SCS layer 130 to beprecisely fixed, at for example, 30 degrees in the embodiment shown inFIG. 1. Torsional mirror 150 is electrostatically actuated by actuationelectrodes 120 to perform, for example, an optical scan.

FIG. 2 shows a top view of one combination deflection mirror/torsionalmirror solid state element. Hinge 155 and deflecting mirror 140 is shownalong with hole 165 to receive the tab (not shown) on support latch 168.The layout of torsional mirror 150 supported by torsion bar 170 withrespect to hole 117 is also shown.

FIG. 3a shows an alternative embodiment in accordance with the presentinvention which requires via 310 for laser beam 199. Via 310 has across-sectional area much smaller than that of VCSEL 105. This preservesSCS layer 130 and other MEMS layers opposite VCSEL 105 for potential usein forming MEMS devices. In the embodiment in FIG. 3a, alignment recess135 is on the back side of wafer 100, opposite the surface MEMS layers.VCSEL chip 105 is solder bump bonded to wettable metal bonding pads 111on PSG layer 119 on wafer 115. Light passes through aperture 310 formedby wet etching and/or deep RIE. Torsional mirror 150 iselectrostatically actuated by actuation electrodes 120 to perform, forexample, an optical scan.

FIG. 3b shows an alternative embodiment in accordance with the presentinvention similar to that shown in FIG. 1. However, the embodiment shownin FIG. 3b has ferromagnetic thin film 330 deposited on torsional mirror150 and thin film coil 340 deposited on glass or SiO₂ coated Sisubstrate 101. Magnetization of ferromagnetic thin film 330 is in theplane of torsional mirror 150 so that magnetic field 320 generated bythin film coil 340 will actuate torsional mirror 150.

FIG. 3c shows an alternative embodiment in accordance with the presentinvention similar to that shown in FIG. 1. However, the embodiment shownin FIG. 3c has microfabricated metal thin film coil 350 with a diameterapproximately that of torsional mirror 150 deposited on torsional mirror150. Metal thin film coil 350 generates magnetic field 360 (shown forcounterclockwise current flow in thin film coil 350) perpendicular totorsional mirror 150 when current is passed through thin film coil 350.Additionally, external magnetic field 370 parallel to torsional mirror150 is present. Depending on the direction of the current flow in thinfilm coil 350, torsional mirror 150 will rotate to the left or to theright in FIG. 3c to minimize the misalignment between magnetic field 360and magnetic field 370.

Steps for fabricating deflecting mirror, supporting latch and VCSEL inaccordance with this invention are shown in FIGS. 4a-4j. The startingmaterial used as a substrate is typically a silicon on insulator (SOI)wafer. Such silicon wafers are commercially available from severalmanufacturers such as Bondtronix, Inc. of Alamo, Calif. and IbisTechnology Corporation of Danvers, Mass. Typically, the thickness of SCSlayer 130 is chosen to be 2-20 μm depending on the stiffness that isrequired of the torsional spring elements and the mirror surfaces to beconstructed from MEMS layer 130. Other mechanical layers are depositedon top of SOI wafer 100 by well-known methods such as Low PressureChemical Vapor Deposition (LPCVD). The deposited layers are mechanicallayers of polycrystalline silicon (poly) and a sacrificial oxide layerthat is phosphorus-doped glass (PSG). Aluminum can be deposited bysputtering or thermal evaporation. FIG. 4c shows PSG layer 119 of 1-2 μmthickness directly on top of MEMS layer 130 of SOI wafer 100.

FIGS. 4a-n show the processing steps used to fabricate deflecting mirror140, supporting latch 168 and VCSEL 105 in an embodiment in accordancewith this invention. Supporting latch 168 has a tab (not shown) whichinserts into corresponding hole 165 (see FIG. 2) in deflecting mirror140. Deflecting mirror 140 and supporting latch 168 are defined byreactive ion etching (RIE) using CF₄ with 4-10 percent O₂ during theetching steps. The completed deflecting mirror 140 and supporting latch168 configuration is shown in FIG. 4n. The typical size of deflectingmirror 140 is in the range of 0.5-1.0 mm square.

FIG. 4a has SiN_(x) layer (not shown) deposited on SOI wafer 100 byLPCVD. SiN_(x) layer is patterned using CF₄ /O₂ RIE with a photoresistmask to form a mask for KOH (potassium hydroxide) etching of Si. KOHetching is used to etch hole 117 from the bottom of SOI wafer 100,stopping on insulator layer 116 of SOI wafer 100. The dimensions ofetched hole 117 will be comparable to that of torsional mirror 150 toallow free rotation of torsional mirror 150. Alternatively etched hole117 may be defined by deep RIE using C₄ F₈ and SF₆ with a SiN_(x) orphotoresist mask.

FIG. 4b shows SOI wafer 100 with recess 135 (200-250 μm deep) etchedinto SOI wafer 100 using a combination of CF₄ /O₂ RIE for MEMS layer 130and insulator layer 116 and deep RIE using C₄ F₈ and SF₆ in substrate115.

FIG. 4c shows chemical vapor deposition (CVD) of phosphorus-doped glass(PSG) 119.

FIG. 4d shows a wet etch using hydrofluoric acid of windows 410 and 420in PSG layer 119.

FIG. 4e shows deposition of aluminum film 430 (0.1-0.2 μm) as a highreflectivity layer.

FIG. 4f shows a wet etch (typically a mixture of phosphoric and nitricacid) Al to leave Al in mirror regions.

FIG. 4g shows etching of vias 133 to Si substrate 115 using CF₄ /O₂ RIEwith a photoresist mask.

FIG. 4h shows the deposited polysilicon layer of 1-2 μm thickness afterbeing patterned to form hinge 155 for deflecting mirror 140. Patterningof polysilicon hinges 155 is described in Wu, "Micromachining forOptical and Optoelectronic Systems", Proceedings of IEEE, vol. 85,p.1833, 1997 and Pister et al., "Microfabricated hinges", Sensors andActuators, A: Physica v 33 n 3 p. 249-256, 1992 which are herebyincorporated by reference in their entirety. If the RIE etching step isdone before deposition of the polysilicon layer, the polysilicon can bedeposited in etched recess 135 to reduce surface roughness due to theetching.

FIG. 4i shows the etch of PSG layer 119 and SCS layer 130 to patterndeflecting mirror 140, hinge 155 and access holes 137. Holes 137 allowfor the etchant used to release deflecting mirror 140 to reachinsulating layer 116. A typical size for holes 137 is 10 μm by 10 μm.Torsional mirror 150 is also defined in this step. Typical size fortorsional mirror 150 in accordance with this invention is in the rangeof 1-2 mm square.

FIG. 4j shows the Ti--Au deposition of wettable metal bonding pads 111and solder for solder bumps 110. Solder is reflowed into solder bumps110 by heating at temperatures<310° C. This leaves the finished,unreleased MEMS parts, along with precisely defined recess 135, readyfor the GaAs bonding step in FIG. 4.

FIG. 4k shows release of deflecting mirror 140 and hinge 155 by etchingPSG layer 119 and insulator layer 116 by using a hydrofluoric (HF) basedetch.

FIG. 4l shows placement of VCSEL 105 (thickness from 100-125 μm) intorecess 135 for the GaAs bonding step. Solder bumps 110 can be defined onVCSEL 105 and VCSEL 105 is placed into recess 135 which approximatelyaligns the bumps to wettable metal bonding pads 111 and 113 due to thecoordinated geometry of VCSEL 105, recess 135, wettable metal bondingpad 111 and solder bump 110 positions. Si Substrate 115 and VCSELsubstrate 106 are heated to allow solder to flow and contact wettablemetal bonding pads 113 on the bottom of VCSEL substrate 106.

FIG. 4m shows hinges 155, deflecting mirror 140, torsional mirror 150and VCSEL 105 bonded to glass substrate 101 or to SiN_(x) -coated orSiO₂ -coated Si substrate 101. Substrate 101 supports actuationelectrodes 120 for torsional mirror 150.

FIG. 4n shows raised deflecting mirror 140 locked with latch 168. Angleof deflecting mirror 140 is fixed by the length of latch 168 andposition of hole 165 at base of deflecting mirror 140.

Linear arrays of lasers can be bonded in a similar way; the extent ofthe array being perpendicular to the cross section shown in FIGS. 3a and3b.

While the invention has been described in conjunction with specificembodiments, it is evident to those skilled in the art that manyalternatives, modifications, and variations will be apparent in light ofthe foregoing description. Accordingly, the invention is intended toembrace all other such alternatives, modifications, and variations thatfall within the spirit and scope of the appended claims.

What is claimed is:
 1. An integrated laser beam scanning structurecomprising:a wafer having a recess on a side; a layer having a firstregion and a second region, said layer being attached to said side ofsaid wafer having said recess; a deflecting mirror fashioned from saidfirst region of said layer; a torsional mirror fashioned from saidsecond region of said layer, said torsional mirror having a first side;and a semiconductor light emitter mounted in said recess whereby a lightbeam emitted from said semiconductor light emitter is deflected by saiddeflecting mirror onto said first side of said torsional mirror.
 2. Thestructure of claim 1 wherein said wafer is a silicon on oxide wafer. 3.The structure of claim 1 wherein said layer is a single crystal siliconlayer.
 4. The structure of claim 1 wherein said semiconductor lightemitter is mounted in said recess using solder bumps.
 5. The structureof claim 1 wherein said semiconductor light emitter is a VCSEL chip. 6.The structure of claim 1 wherein said recess is deep reactive ionetched.
 7. The structure of claim 1 wherein said torsional mirror isactuated by a pair of electrodes.
 8. The structure of claim 1 wherein aferromagnetic thin film is attached to said first side of said torsionalmirror.
 9. The structure of claim 8 wherein said torsional mirror isactuated by a thin film coil.
 10. The structure of claim 1 wherein athin film coil is attached to said first side of said torsional mirror.11. A method for making an integrated laser beam scanner comprising thesteps of:providing a wafer having a recess on a side; attaching a layerhaving a first region and a second region to said side of said waferhaving said recess; fashioning a deflecting mirror from said firstregion of said layer; fashioning a torsional mirror from said secondregion of said layer, said torsional mirror having a first side; andmounting a semiconductor light emitter in said recess such that a lightbeam emitted from said semiconductor light emitter is deflected by saiddeflecting mirror onto said first side of said torsional mirror.
 12. Themethod of claim 11 wherein said wafer is a silicon on oxide wafer. 13.The method of claim 11 wherein said layer is a single crystallinesilicon layer.
 14. The method of claim 11 wherein said semiconductorlight emitter is mounted in said recess using solder bumps.
 15. Themethod of claim 11 wherein said semiconductor light emftter is a VCSELchip.
 16. The method of claim 11 wherein said recess is deep reactiveion etched.
 17. The method of claim 11 wherein said torsional mirror isactuated by a pair of electrodes.
 18. The method of claim 11 whereinsaid torsional mirror is actuated by a thin film coil and an externalmagnetic field.
 19. The method of claim 11 wherein a ferromagnetic thinfilm is attached to said first side of said torsional mirror.
 20. Themethod of claim 11 wherein a thin film coil is attached to said firstside of said torsional mirror.